Most genes involved in DNA replication in the yeast Saccharomyces cerevisiae are transcribed transiently during late G1 as cells undergo START. Their promoters all contain one or more versions of an 8-base pair motif (ACGCGTNA) called the MluI cell cycle box (MCB). MCBs have been shown to be both necessary and sufficient for the late G1-specific transcription of the TMP1 thymidylate synthase and POLI DNA polymerase genes. A different late G1-specific transcription element called the SCB (CACGAAAA) is bound by a factor containing the SWI4 and SWI6 proteins. We describe here the formation in vitro of complexes on TMP1 MCBs that contain the SWI6 protein and, we suggest, a 120 kDa protein that is distinct from SWI4. Transcription due to SCBs and MCBs occurs in the absence of SWI6 but it is no longer correctly cell cycle regulated. We suggest that SWI6 is an essential regulatory subunit of two different START-dependent transcription factors. One factor (SBF) contains SWI4 and binds to SCBs whereas the other (MBF) contains p120 and binds MCBs.

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